The problem
Space heating and hot water make up most of the energy used in European homes, and much of it still comes from gas and oil boilers. Replacing those boilers with heat pumps powered by an increasingly clean grid is the main way to cut emissions from existing housing. For years, though, heat pumps were seen as a product for new, well-insulated buildings with underfloor heating. Older houses have radiators sized for hot water from a boiler, often at 60°C or more, and many owners were told a heat pump would not keep them warm.
A second problem was the refrigerant. Many heat pumps used fluorinated gases (F-gases) such as R410A, which are powerful greenhouse gases when they leak. The EU’s revised F-gas Regulation now bans new self-contained (monoblock) heat pumps up to 12 kW that use F-gases with a global warming potential of 150 or more from 1 January 2027, unless safety rules require otherwise (European Commission, F-gas alternatives).
The product
Vaillant tackled both issues with the aroTHERM plus. It was presented at the ISH trade fair in 2019 and went on sale in April 2020, with 5, 7 and 12 kW models first and 3 and 10 kW models from June 2020 (Vaillant, 10 March 2020). It was Vaillant’s first heat pump designed to work in existing buildings as well as new ones. The company said it could supply a typical radiator system at 55°C without a second heat source, and reach 75°C for hot water, which removes the need for an electric immersion heater to protect against legionella.
The UK installer data sheet lists flow temperatures from 20 to 75°C, operation down to minus 25°C, an R290 charge of 0.9 kg in the 7 kW unit and a SCOP of up to 5.03 (Vaillant UK, aroTHERM plus 7 kW). Vaillant reports savings of up to 43% in retrofit tests when the unit replaced oil or LPG heating.
In March 2025 Vaillant showed a fully redesigned aroTHERM plus at ISH 2025, and it went on sale in Germany and Austria in September 2025, again from 3 to 12 kW (Vaillant, 1 September 2025). Vaillant gives a seasonal space heating efficiency of up to 202%, a night-mode sound level of 27 dB(A) at three metres for the 3 kW unit, and a new safety concept that lets the outdoor unit sit closer to doors, cellar windows and light wells. Vaillant describes the aroTHERM plus as one of Germany’s best-selling heat pumps. Current product data list a low-temperature SCOP of 4.83 to 5.2 and a medium-temperature SCOP of 3.62 to 3.94 across the range (Vaillant product page).
How it works
A heat pump moves heat rather than making it. In the aroTHERM plus:
- Evaporator: a fan pulls outdoor air across a coil. The cold liquid propane inside absorbs heat from the air and evaporates, even when it is well below freezing outside.
- Compressor: an inverter-driven compressor squeezes the propane vapour, raising its temperature.
- Condenser: the hot refrigerant passes its heat to the water circuit that feeds radiators, underfloor heating and the hot water cylinder.
- Expansion valve: the refrigerant drops in pressure, cools, and the cycle starts again.
Because it is a monoblock, the whole refrigerant circuit is sealed in the outdoor unit at the factory. Installers only connect water pipes and electrics, so they do not need refrigerant handling certification. Propane suits higher temperatures well, which is why the unit can reach 75°C. It is flammable, though, so units must respect safety zones around openings; the 2025 model was designed to shrink those zones. The unit can also cool in summer.
Timeline
| Date | Milestone |
|---|---|
| 2013 | Vaillant launches the first aroTHERM with a modulating inverter compressor |
| Mar 2019 | aroTHERM plus with R290 unveiled at ISH, Frankfurt |
| Apr 2020 | Sales start (5, 7, 12 kW); 3 and 10 kW from June 2020 |
| Feb 2024 | Revised EU F-gas Regulation adopted |
| Mar 2025 | New-generation aroTHERM plus shown at ISH 2025 |
| Sep 2025 | New generation available in Germany and Austria |
| 1 Jan 2027 | EU ban on high-GWP F-gases in new monoblock heat pumps up to 12 kW |
Impact and numbers
- Market: the European Heat Pump Association counted about 2.9 million residential heat pumps sold in 21 European countries in 2025, up 13% on 2024, with Germany up 50%; roughly 29.3 million are now installed (EHPA market data). Sales in 12 countries rose another 11% in the first half of 2026, to about 1.16 million units (EHPA, 2026).
- Real homes: Fraunhofer ISE measured heat pumps in existing single-family houses in Germany. For 49 air-source units, seasonal performance factors ranged from 2.6 to 4.9 and averaged 3.4, with heating circuit temperature, not building age, the main driver (Fraunhofer ISE).
- Subsidy: under Germany’s new Building Modernisation Act, KfW grants for heat pumps restarted on 21 July 2026 with a 30% base rate, a 16% “climate speed” bonus and income bonuses of up to 40%, on eligible costs of up to €28,000. From early 2027 the base rate falls to 15%, with a 15-point bonus for heat pumps made in the EU (KfW, 21 July 2026).
Honest caveats. SCOP figures are laboratory ratings at defined temperatures; real performance depends on flow temperature, radiator sizing, hydraulic balancing and installer skill, as the Fraunhofer spread shows. Running costs depend on the ratio of electricity to gas prices, which remains unfavourable in several EU countries. Propane is flammable, so siting rules matter. Savings claims such as “up to 43%” come from Vaillant’s own tests. Heat pump markets have also swung with policy: sales fell sharply in 2024 before recovering, and Germany’s subsidy rules have changed again in 2026.
What’s next
The 2027 F-gas deadline pushes the whole industry towards natural refrigerants, which plays to products already designed for propane. Digital services are the other front: Vaillant offers optimisation software that adjusts settings automatically, and an energy management service that runs the heat pump on rooftop solar power or cheap hours on a dynamic tariff. The planned German bonus for EU-made units, if introduced as announced, would favour European manufacturing.
Why it matters for Europe / green buyers
For homeowners, housing associations and installers, the aroTHERM plus is a mature, mass-market product that shows radiators do not rule out a heat pump. Buyers should ask for a room-by-room heat loss calculation, target the lowest flow temperature that keeps rooms comfortable, check outdoor siting clearances for propane units, and compare quotes on expected seasonal performance, not just price.
Heat pumps matter beyond Europe too. In India and other hot countries, the same technology is in every air conditioner, and heat pump water heaters are starting to replace electric geysers in hotels and hospitals.
Sources & image credits
- Vaillant, “aroTHERM plus air-to-water heat pump” product page: https://www.vaillant.com/en/products/arothermplus/
- Vaillant Group, “Sales launch: new aroTHERM plus heat pump”, 10 March 2020: https://www.vaillant-group.com/news-stories/sales-launch-new-arotherm-plus-heat-pump.html
- Vaillant Group, “More efficient, quieter, and flexible installation options: new heat pump aroTHERM plus”, 1 September 2025: https://www.vaillant-group.com/news-stories/more-efficient-quieter-and-flexible-installation-options.html
- Vaillant UK, “aroTHERM plus 7 kW air source heat pump” installer page: https://professional.vaillant.co.uk/for-installers/products/arotherm-plus-air-source-heat-pump-7kw-146561.html
- European Commission, “Air conditioning: climate-friendly alternatives to F-gases”: https://climate.ec.europa.eu/eu-action/fluorinated-greenhouse-gases/climate-friendly-alternatives-f-gases/air-conditioning_en
- European Heat Pump Association, “Market data”: https://ehpa.org/market-data/
- European Heat Pump Association, “Post-Hormuz heat pump sales climb 11%”, 2026: https://ehpa.org/news-and-resources/news/post-hormuz-heat-pump-sales-climb-11/
- Fraunhofer ISE, “HP-QA in existing buildings (WP-QS im Bestand)”: https://www.ise.fraunhofer.de/en/research-projects/wp-qs-im-bestand.html
- KfW, “New Federal Funding for Efficient Buildings programme starts today”, 21 July 2026: https://www.kfw.de/About-KfW/Newsroom/Latest-News/Pressemitteilungen-Details_901760.html
Images:
- “Air Source Heat Pump - Vailliant aroTherm Plus on a terraced house” by Southend-on-Sea Borough Council, released under CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/), via Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Air_Source_Heat_Pump_-_Vailliant_aroTherm_Plus_on_a_terraced_house.jpg
